1. Field Of The Invention
The present invention relates to orthopedic prosthetic devices and more particularly relates to a modular joint prosthesis. Even more particularly, the present invention relates to an improved orthopedic joint implant that allows the surgeon to choose intraoperatively, the valgus angle for a stem extension that will best fit the patient's intramedullary canal by using a stem member having two tapers, one at each end of the stem and each taper having a different angle in relation to the central longitudinal axis of the stem itself. The apparatus of the present invention thus provides an easy method and apparatus for selecting the valgus angle, simply using one end or the other depending on the desired angle.
2. General Background
Most orthopedic implants that have an extension that fits into the intramedullary canal of a patient's bone provide a fixed position for the extension relative to the joint prosthesis that can not be changed intraoperatively. However, some prior art type joint implants do provide variable valgus angles. The prior art typically solves this problem by placing the variable angle structure at one end of the stem only. By only providing a variability at one end of the stem, a complicated system is necessarily required in order to change the angle. Many different components and/or complicated linkage can be required to achieve the desired result.
Many joint prosthetic devices have been patented which have discussed the problem of changing the valgus angle intraoperatively.
U.S. Pat. No. 5,152,796 uses a series of bolts extending through a femoral component at different angles that will correspond to different valgus angles. The loosening of a bolt could lead to catastrophic failure of the entire component. Any prosthesis system of the '796 patent includes stems of different lengths and different diameters which can be attached to the bolts to provide a system with stems of different and diameter lengths set at different valgus angles.
U.S. Pat. No. 5,133,760 provides a modular prosthesis stem extension that is discussed as being an improvement over U.S. Pat. No. 4,985,037. A stem extension is disclosed with a six degree valgus inclination as well as a degree of superior or inferior angulation such as for example two (2.degree.) degrees. This inclination and angulation are anatomically correct in each reversed position of the modular stem extension. The '760 patent discloses a coupling mechanism and a stem with an off-set angle at one end that goes through the coupling. This allows the stem to be rotated at different positions for different angles, then locked into place with a locking nut. There is a possibility of loosening at the locking mechanism which could lead to instability and failure of the prosthesis.
U.S. Pat. No. 5,002,581 issued to Paxxon entitled "Modular Hip Joint Prosthesis with Adjustable Anteversion" provides a modular hip prosthesis and instrumentation for implanting same. The hip prosthesis has provision for varying the angulation between the stem portion and the trochanteral module portion by provision of connection means between the neck and stem which can be positioned or attached together in a variety of rotational positions. Variation of the angulation or anteversion is made possible in the virtue of the fact that the axis of the connection portion of the stem and neck is angularly off-set from the axis of the body of the stem and neck respectively. Instrumentation is provided for formation of a cavity for implantation of a prosthesis which is provided within indication means for indicating to the surgeon the optimum angle for the assembly of the prosthesis for implantation into a particular proximal femur. A problem can occur with this system when rotating the stem as translation of the stem is not only in the desired medial-lateral direction, but also to the anterior or posterior direction, depending on the angle desired. This could result in impingement of the stem with the cortex and the canal in the anterior or posterior position.
U.S. Pat. No. 4,985,037 issued to Peterson provides a universal modular prosthesis stem extension designed to be attachable on the under surface of a femoral or tibial prosthesis. The stem extension forms part of a modular system wherein a single stem extension could be selectively attached to any one of a multiplicity of femoral or tibial components, thus reducing the amount of inventory which is necessary to be maintained by a surgeon. In the preferred embodiment, each stem extension is designed with a six (6.degree.) degree valgus inclination and is reversible for left or right patients. The system is held in place with pins that could loosen during use. Because a single stem must be required for each different angle, increased inventory is required.
U.S. Pat. No. 4,822,366 issued to Bolesky provides a modular knee prosthesis assembly for the replacement of a portion of the knee joint. The assembly has a femoral component which is formed to include first bearing surfaces and first means for demountably receiving at optional femoral system member. The assembly also has a tibial component that is formed to include a platform and second means for demountably receiving an optional tibial stem member. The assembly includes a separate tibial insert that is configured to be supported by the tibial component platform. The tibial insert is formed to include second bearing surfaces that are configured to mate with the first surfaces on the femoral component to permit pivotal movement between the femoral component and the tibial component. This system puts a stem with the reduced diameter into a mating counter bore of a mounting fixture which is then affixed to the component by means of a screw or nut on the side of the component opposite the stem mounting fixture. As the screw or nut is tightened the component is sandwiched between the screw or nut in the stem mounting fixture. A problem can occur in that this system provides no locking mechanism to prevent rotation of the stem which in turn can cause loosening of the screw or nut or loss of fixation of the entire component.
A French patent 0545833A1 allows the stem to be shifted to align with an intramedullary canal property. However, the French '833 patent does not provide variable valgus angle as provided by the present invention.